Category Archives: Geochronology specific issues

To determine the rate of ice sheet retreat it is necessary to determine WHERE the ice sheet margin was WHEN. The WHERE is determined by studying landforms characteristic of glacier margins, such as moraines, erratic boulders, and sediment deposited both onshore and offshore by ice and glacial meltwater near the margins of the ice sheet. The WHEN is determined using different dating techniques depending on the material sampled. For BRITICE-CHRONO we are applying three different dating techniques, (1) optically stimulated luminescence dating (OSL) for sands, (2) radiocarbon dating (14C) for organic material like shell fragments picked out of sediment cores collected from the sea floor during the research cruises, and (3) surface exposure dating of rock using the terrestrial cosmogenic isotope beryllium-10 (10Be) in the mineral quartz.

Radiocarbon dating and surface exposure dating rely on being able to measure the abundance of extremely rare radioisotopes in the sample material using a technique called accelerator mass spectrometry (AMS) performed at the Scottish Universities Environmental Research Centre (SUERC) AMS Laboratory. What is extremely rare? Well, the natural abundance of 14C in modern carbon is 1 part per trillion (10-12) which gets smaller with the age of the material being dated because 14C decays after the death of the organism. The ratio between the radioisotope 10Be and stable beryllium is roughly 10-13 to 10-14 for surface exposure dating of BRITICE-CHRONO samples. To put this in perspective, if you counted at one number per second it would take you about 3.25 million years to count to 1014. In other words, radiocarbon and surface exposure dating is only possible because we are able to count individual radioisotopes among trillions of almost identical stable isotopes.

In scientific papers these measurements are often summarised in a few words, such as “…[samples were] reduced to graphite before AMS 14C analysis at the SUERC AMS Facility” or “10Be/9Be ratios were derived from measurements at the SUERC AMS Laboratory”. Neither sentence does justice to the complexity of the method and the efforts of a dedicated team of scientists and technicians. Here I will try to provide some insight into how the concentrations of the extremely low quantities of 14C are determined using accelerator mass spectrometry (AMS). For 10Be the process is similar but explaining the differences would add unnecessary complexity in what follows.

Accelerator mass spectrometry (AMS) is an ultra-sensitive technique for isotopic analysis in which atoms extracted from a sample are ionized (the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons to form ions); accelerated to high energies; separated according to their momentum, charge, and energy; and then individually counted after identification as having the correct atomic number and mass. The principle difference between AMS and conventional mass spectrometry (MS) lies in the energies to which the ions are accelerated. In MS the energies are thousands of electron volts (1 keV = 1.6 x 10-16 J), whereas in AMS they are millions of electron volts (MeV). The practical consequences of having higher energies is that ambiguities in identification of atomic and molecular ions with the same mass are removed.

This is how we make the measurements at SUERC

Fig 1. Cathodes in sample wheel after measurement. The hole in the cathodes contains the sample and is 1mm wide.

Samples come to the AMS Laboratory in the form of pressed cathodes containing a few milligrams of graphite (for radiocarbon) and BeO mixed with Niobium for 10Be surface exposure dating. It takes a lot of time and effort to get from a sample collected in the field to the cathode stage, but that is another story.

The cathodes are loaded into a 134-position sample wheel together with standard materials that have known isotope ratios.

Fig 2. Sample wheel loaded in ion source.

The wheel is loaded into the ion source, the ion source is closed and pumped down to the same very high vacuum as the remainder of the beam line (steel tube) through which the ions produced in the ion source are going to travel (the correct term is drift, but it does not really convey the speed at which the ions move). We need a very high vacuum (comparable to conditions found in outer space) because we do not want our ions to collide with neutral atoms or molecules drifting anywhere along the 32 metres of beam line.

Fig 3. Section through ionizer

We are now ready to perform some ion sorcery. We heat up a caesium reservoir to generate a Cs vapour in the space between the cathode holding our sample and a heated ionizing surface. Some of the vapour condenses onto the cooled cathode, some is ionised by the ionizer creating Cs+ ions that are accelerated and focused towards the sample because the sample cathode is at -5kV compared to the ionizer (Fig 4). The impact of the Cs+ ions on the sample surface causes sputtering of particles from the sample surface.

Fig 4. Cs+ focus lens at front of ioniser (Fig. 3). The sample cathode being ionised is located 1 mm in front of the hole in the lens

Some materials will preferentially sputter negative ions. Other materials will preferentially sputter neutral or positive particles, which pick up electrons as they pass through the condensed caesium layer, producing negative ions. All of this happens within 1 mm of the sample surface. The negative ions are removed from the sample surface and focused into the beam line by an extraction electrode (extractor) set at 15 kV. The extracted ion beam (shown in grey in Fig. 3) is spreading, just like the spreading of light from a torch (which is just another type of particle beam). This is known as beam emittance and it must be kept small to ensure high transmission to the detector. But I am getting way ahead of myself safe to say that we cannot get the emittance of the beam back to the original 1mm diameter at the sample surface and much of the experimental apparatus of the AMS is designed to focus the beam to specific places along the beam line.

Fig 5. Closed ion source. The metal rings around the beam line are part of the 45 kV injector. The apparatus to the left of the rings are for pumping the beam line and and focussing the beam.

So now we have extracted a negative ion beam from our sample material, and the ions are starting to move along the beam line. We accelerate the particles to 66 keV by exposing them to an additional 45kV in the injector (Fig. 5 and Fig 6). The particles pass through a spherical electrostatic analyser (ESA, Fig. 6), where particles with the incorrect energy over charge (E/q) are removed from the beam, before being injected into the accelerator via the injection magnet.

Fig 6. Schematic of SUERC 5MV tandem spectrometer.

The injection magnet separates particles based on momentum (= mass x velocity). For radiocarbon we set the magnetic field to allow particles with mass 14 through the magnet and into the accelerator, but we also need to put carbon-12 and carbon-13 into the accelerator to get the ratio 14C/12C and 14C/13C. Since both 12C and 13C have lower mass than 14C we use a magnet bouncing system (MBS; Fig. 6) to give the ion beam more energy so that 12C and 13C temporarily behave like 14C. Because 12C and 13C are much more abundant than 14C we inject the former two for only a few microseconds per second. While 14C enters the accelerator we measure the 13C current in a low energy Faraday cup and the same is the case for 12C when 13C is injected (Fig. 6. Inset A). 14C cannot be measured in a Faraday cup because there are far too few atoms to generate a current.

Unfortunately sample materials are not pure and therefore ion sources do not only produce the ions we want. However the next stage in the process takes care of many of the molecular isobars (= same atomic mass number) such as the hydrocarbons 12CH2– and 13CH–, which have the same atomic mass number as 14C and are therefore also injected into the accelerator.

So far the system described is similar to conventional mass spectrometry. The next stage in the ion transport is what sets AMS apart. Up to now the ions have been energised by the ion source and injector to 66keV (the low energy end of the spectrometer), which means they are travelling at roughly 1000 km/s. Next they are accelerated to a speed of roughly 7500 km/s by exposing the negatively charged ions to a 4.5 MV positive charge at the terminal in the centre of the 8 m long accelerator tank (Fig. 6 & 7).

Fig 7. Accelerator pressure vessel known as the tank. It is filled with an insulating gas.

When the negatively charged ions reach the terminal they pass through a gas stripper (Fig. 6). The collisions between the ions and the gas removes electrons from the ions thereby changing them from being negatively charged to being positively charged. The terminal voltage is set to remove at least three electrons because by this process molecular isobars of 14C (such as 12CH2– or 13CH–) are destroyed due to the high instability of their positively charged forms, and atomic C+ ions such as 12C+, 13C+, and 14C+ can be separated due to their different mass to charge ratios. Once the now positive atomic ions emerge from the stripper they find themselves next to a very high positive charge (4.5 MV in the case of radiocarbon) and they accelerate away from this, hence the term tandem accelerator (two acceleration steps). The particles emerge from the tank at a velocity of greater than 17000 km/s (that’s equivalent to traveling around the Earth at the equator in just over 2 seconds), the high-energy part of the AMS.

The particles now enter another mass spectrometer, the analysing magnet (Fig. 6), where the 12C+, 13C+, and 14C+ are separated according to momentum. 12C+ and 13C+ currents are bent more than 14C+ and are collected and measured in Faraday cups, while 14C+ is allowed to continue on towards the gas ionisation detector. Even after all of this the ion beam still contains ions with incorrect mass, energy, or charge as a result of energy- or charge-changing collisions with system components or residual gas. Unfortunately these interferences mimic the path of the ions of interest. Some of them are removed by another electrostatic analyser (ECA) before the remaining particles arrive at the gas ionisation detector where the final identification and counting of atoms takes place (Fig. 8).

Fig 8. Detector (foreground) and electrostatic analyser (background).

As the ions enter the detector they are slowed down and stopped by passing through a gas. Each atom ionises some of the gas and the resulting electrons are collected, amplified, and digitised. In this way the path and location of each atom arriving in the detector can be determined and each arrival counted. The gas ionisation detector allows us to determine the atom species because heavier atoms travel further and deposit more energy than lighter atoms. This capability allows us to set the electronics to separately count 14C atoms and different interferences arriving in the detector (Fig 9).

Fig 9. Detector spectrum of one 6 minute measurement. Red dots (n=40933) are counted 14C events. Black dots (n=430) are scattered 14C and Lithium atoms (labelled) that have made it into the detector.

To make all of this possible requires every component of the accelerator mass spectrometer to operate together within very tight tolerances. Even when everything is working well, each time a new sample wheel is introduced into the ion source the conditions inside the ion source vary slightly and the machine has to be very carefully tuned to these new conditions prior to commencing the measurement of the precious samples. Once satisfied the AMS is operating within the necessary limits each sample is measured until measurement statistics are met (usually within 8 measurements), or the sample is exhausted. Each individual measurement lasts about 6 minutes. For high precision measurements it is not unusual to measure a sample for a total of 1 to 1.5 hours. Thus to complete the measurement of a sample wheel is a multi-day undertaking during which the AMS has to be continually monitored for any changes in the condition of multiple machine components that could compromise measurement integrity.

The 14C/13C ratio for each measurement is derived from the counted 14C atoms in the sample divided by the number of 13C atoms calculated from the 13C current in the high energy Faraday cup. The final 14C/13C ratio for each sample is the average of the combined 14C/13C ratio measurement for the sample, normalised to known standard materials that were measured in the same wheel. It is this final 14C/13C ratio that is used to calculate the radiocarbon age for the sample.

I hope the above summary provides a little bit of insight into what is meant when you come across sentences like “…AMS 14C analysis were made at the SUERC AMS Facility” in the literature.

After a decade of dreaming and years of planning our team of 40 data-hungry geoscientists were given the scent and released from their cages (~desks) with the audacious task of blitzing the whole ice sheet to find samples for dating its retreat. This started in November 2012 in a grey drizzle at Seisdon sand and gravel quarry near Stourport and finished 09:30am 1st August 2015 in bright sunshine when we extracted our last sample, a seafloor core, from the Cleaver Bank in the southern North Sea. It really has been an epic two and half years witnessing the Terrestrial Team with sun-cream in the Scilly Isles to shivers in Shetland, and with dressing gowns in Donegal to JCBs in Norfolk. We really did covered the ground from south to north and east to west and snuck in 28 – yes 28 – different islands of Britain and Ireland, including Scilly Rock and Foula. When samples were not easy to spot and grab, we used radar, seismics and some occasional guesses to work out where to dig with shovel or digger or to core the hidden sediments. It is not quite true that no stone was left unturned, but I have been amazed at how close we got to that, thanks to some amazing levels of energy and motivation; it is indeed lucky that our team displayed traits of obsessiveness and kleptomania when it came to sampling. Bloody well done to all.

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So our very last sample (core 179-VC) on BRITICE-CHRONO has now been collected, marking the end of Cruise Two on RRS James Cook. Even though we never got to shout ‘One hundred and …eighty’ it is more than we had planned. We have sailed, steamed, or dieseled 8971.65 kilometres, taking in Skye, Rona, Shetland, and more North Sea banks including (the infamous Dogger) that you could shake a stick at. We have sampled deep (525 m) and very shallow (19 m), and calm and troubled (force 7). Our ship-track might look erratic to some but, as they say in marketing non-speak, it comprises a subtle blend of caution and well-planned targets with a hint of adventure and wild abandon yielding a truly inspiring collection of mud and sand to sate the yearnings of the most inquisitive discerners of ice sheet curios.

The loot under the care of Team Marine (Lou and Margot)

The haul, now sat in our refrigerated lorry-container and packed in plastic tubes was obtained by lowering our vibro- and piston corers through 18,891.4 metres of seawater and extracting over half a kilometre of sediment (Rich says 542.4 m). As well-known, of course, it is not the length that counts, but the quality. It will be some time however before we know which cores, places and transects yield the best shells and forams for dating, but Margot and Lou have already bagged, sifted and labelled the celebrity shells which we think have the best stories to tell….’well there was this bloomin’ huge great wall of ice that kept crashing down, and would you believe what happened next….’.

Science crew of the RRS James Cook cruise JC123

Thanks to Colm and his science team, the Captain and crew and the geological survey coring teams, and the weather, some good planning, crazy hunches and some luck, this scientific cruise has been a great and enjoyable success. We have a mammoth payload that we hope will provide a legacy of new information for decades. It has been a pleasure having Alex, the ever-present black ninja-photographer on-board, – he stalks, clicks and then runs – in his quest to document our highs, lows and silly moments. Hopefully you have already seen much of his work.

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We set out to do 50 years work in five. Taking this cruise with last year’s, which circumnavigated Ireland, along with our >300 person-days of terrestrial fieldwork we have bagged around 15 tonnes of samples for dating and I hope you agree that we have been around a bit. Sorry if we missed your patch, why don’t you have a go? It is an end of an era for our sampling effort. As project leader, I now breathe a large sigh of relief that it is over and has gone so well, phew and phew again. There is a tinge of sadness though, that we all feel as the fun, bonhomie and making of new friends on hard-won field exploits has now ended. No more pie shops or sneaky pints. Team Terrestrial (Rich and his gang) and Colm’s Marine Crew, can now stand-down to great applause. Derek’s Geochron Team have their work cut out to carefully analyse all the samples and then our Transect Leaders (Tom, Dave, Rich, James, Colm, and Sara) will rise to the challenge of making sense of it all and telling us the story that the shell started to blurt out.

Taking things one day at a time

Chris Clark, signing off on behalf of BRITICE-CHRONO, currently steaming 11 knots, homeward bound, over the Tea Kettle Bank of the southern North Sea. All cores logged and packed and the pinging geophysics finally turned off.

Darkness. A great mass of ice overhead. The eerie rumbling of a large, uncompromising mass, slowly but steadily on the move. Below a thick layer of stiff red sediment, ground off the red bedrock, crushed and churned into a lumpy, sticky blanket of glacial till.

Dark coasts

What would later be called Cape Wrath was only miles to the south, but there was no cape yet. Just the grinding of slow and unforgiving ice moving north into the North Atlantic. But the times were changing. The sun gained in strength, atmosphere and ocean started to warm and the gigantic ice mass, later to be known as the British-Irish Ice Sheet, was in decline. As its surface melted, more water reached its bed, and it began to slide helplessly over its own sediments. Slowly it thinned, and retreated in the direction of the Scottish mountains with the ocean lapping relentlessly at its edges.

There seemed to be no hope, but the ice sheet made one last bold dash towards the edge of the continental shelf before it faltered. The recently deglaciated seabed and freshly deposited grey ocean sediments were bulldozed and overrun again by ice on the move, and buried once more in a blanket of red till. Linear ridges (moraines) marked the limit of this temporary re-advance. But it was only a death throw; the re-advance didn’t get far. The ice sheet’s days were numbered. The advance stopped, and turned into irreversible retreat.

A geophysical search for the perfect core…….

Against a backdrop of rumbling, calving icebergs, station JC123-048VC slowly became ice free, as the snout of the ice sheet moved back over the site. A cold, shallow sea took its place; first, still close to the snout of the ice sheet, where streams of meltwater rushing into the waiting sea water lay down a blanket of coarse sand. As the ice retreated further, taking the meltwater streams with it, the sea fell silent. Only fine sediments spat out into suspension by the dying ice sheet made it to our site, slowly covering it in a thick, grey blanket.

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The ice sheet sent a final message as the ice margins retreated south towards the land; a message from an iceberg. As it passed, melting, overhead of station JC123-048VC, pebbles slipped from its icy grip. They plummeted into the depths, impacting into the soft fine clay sea bed. As soon as this excitement started it was over, and the pebbles were slowly covered by more of the same grey clay.

With the great weight of the ice gone, the Earth’s crust rose like an ancient giant from its slumbers, pushing the Scottish continental shelf closer to the sea surface. Over time, the waters shallowed, and the seabed currents became stronger. The last vestiges of the glacial seafloor were scoured by contour currents, which deposited the spoils of an energetic coast on the eroded sediment below. Millennia later coarse sand and shell debris formed a layer of several inches thick. And then on Sunday the 12th July 2015 all changed.

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There was an unfamiliar thud, and then the uncanny sensation of a vibrating tube burrowing into the sediment from above. It cut through the sand in a jiffy, passed the pebbles, and into the soft clays. The tube slid through it like a hot knife through butter. No struggle with the coarse sands lain down by meltwater streams either, only slowing on reaching the stiff, red till. It battled its way into it for a meter and a half. Then the friction became too much. The vibrocorer stopped, and then the whole tube, now full of sediment, was pulled back up to the sea surface, and hoisted back up onto the deck of the RRS James Cook, the ship it had come from. Peace returned once again on to the sea floor, at core site VC123-048VC, a few miles north of Cape Wrath, on the northwestern edge of Scotland; a land mass now devoid of ice sheets and glaciers.

The core came on board and was cut into sections, labelled, scanned, and split. Finally, we, the scientists who had planned the project, planned the cruise, sailed all the way from Southampton to Cape Wrath, and waited for the British Geological Survey (BGS) to deliver the core, first laid eyes on the sediment. The story was there: a stiff basal till deposited beneath the ice sheet; fines marking the first incursion of the sea; further glacial till documenting the ice re-advance, meltwater stream sediments deposited in front of the retreating ice margin; the fine clays deposited when the ice began to recede southwards containing drop-stones from the icebergs, and the marine sand of the modern seafloor. That was what we had come for. And this was the 48th core; none of the previous 47 had told the story of the vanishing British ice quite this clearly.

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Hopefully we’ll be getting more cores like this in the coming three weeks of the cruise. We need this story told in every sector of the British-Irish continental shelf. Only then will we have what we set out for: the complete saga of the Last British-Irish Ice Sheet.

For Transect 3 of BRITICECHRONO, THE Irish Sea East, north from the terrestrial component in Shropshire-Lancashire, much of the remainder will be be dealt with during the marine cruises. The Isle of Man is the clear exception with excellent terrestrial exposure of the Quaternary geology; it is an excellent candidate region for dating the decline of the ISIS. The Isle of Man occupies a position astride successive ice advances through the Irish Sea Basin and records evidence of fluctuations of ice in the Irish Sea basin. The glacial geology of the Isle of Man is extremely well known, and this knowledge forms the basis for recent BRITICECHRONO fieldwork on the Isle of Man.

Geomorphology of the Isle of Man (Thomas et al., 2006)

Team Isle of Man consisted of Richard Chiverrell, Matt Burke, Daniel Schillereff (all Liverpool University), and David Roberts (Durham University), with meticulous planning (and no hastily rearranged flights) the intrepid team took off for autumnal bedock, erratics, sands, Manx queenies, cliff sections, gravels, sands, buried soils (?), kettlehole basins and ground penetrating radar on 4th to 9th November 2013….. We divided the Island five sectors documenting the northwards retreat, a) the Plains of Malew and adjacent hills (the South); b) the Peel embayment (the Central Valley) and on the northern plain c) outwash deposits of the Shellag Formation (the initial retreat); d) ice marginal sandar deposits associated with the Orrisdale Formation ice marginal oscillations (previously dated by Ian Thrasher) and e) outwash deposits of the Jurby Formations lain down during a more substantial 2-3km readvance. Together geochronology from these sectors would document the phased retreat across the Isle of Man and secure the timing of two well defined readvance episodes (Orrisdale and Jurby events).

Day 1 Monday – Travel and reccie day for some: Roberts, Dave, was first to arrive, apparently having set off before dawn, from whence he set gainfully on reacquainting himself with some former haunts, having spent a happy 12 months on the Island as a post doc in the mid- to late 1990’s. A very good day followed, bedrock sites on the southern flanks of Man, and a search for the famous Foxdale erratic train….. Meanwhile following a 9am lecture to the second years on European peat climate records, Chiverrell (Rich) tried to find his unusually elusive postdoc, Burke (Matt) who had been set the not insignificant challenge of cramming too much equipment into a car that had now seen better days. But second success of the day followed, 2x GPR antennae, 1x RTK Trimble GPS, tripods and staffs, monolith tins, 3x gamma detectors, the Roberts Rocksaw and cosmo kit, luminescence tubes and gearing, plus two scientists, can fit…. Third success, catching the boat from Heysham to Douglas, only 60 mins early for check in this time….. By 10.30 we had all collected in Andreas in the far north of the Islands, via in Dave’s case some old haunts in Douglas and a fine meal in the Sulby Glen Hotel for Matt and Rich.

Day 2 Tuesday – The Plain of Malew: The excellent recognisance by Dave helped us make short work of the very south of the Island. Bedrock samples a quartz arenite and quartz vein (sample 1 and 2) from Cregneash Peninsula overlooking the Calf of Man, where ice skirting the western flank of the Island has scoured and streamlined the topography and permission given by a very helpful landowner. The search for outwash sand and gravels for OSL proved slightly more taxing, with in the late afternoon a former bedrock quarry near Ronaldsway airport, Turkeyland Quarry, yielding a thin outwash deposit (sample 3) and a very enigmatic buried weathered soil, possible 14C target. And a fine dinner of Manx queenies and skate courtesy of chefs Matt and Dave. The final member, Schillereff (Dan), of the team flew in that evening to provide expertise on the kettlehole sediments, and revisit what might have been the locale for his undergraduate dissertation.

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Day 3 Wednesday – The Foxdale Granites and moving northwards:With permission from Manx National Heritage (Isle of Man Government) in order, the ‘holy grail’ site for BriticeChrono was very quickly lined up, the Foxdale granites. Ice flowing north to south penetrated through valleys from Glen Maye and Foxdale valley building to eventually bury and consume the Isle of Man. In Foxdale at the col at the head of the valley (~200m) a granitoid is exposed, and the erratic train holds a place of significance in the geological literature, including the attention of Charles Darwin (1842) as a classic example of transport of glacial boulders from low to higher ground including the summit of South Barrule. With the permission and assistance of Manx National Heritage several boulders were identified on the slopes of South Barrule near an Iron Age hillfort, 260-190m upslope and 1km distant from outcrop (samples 4 and 5). Foxdale granite is quite tough; boy did they take some chiselling. The four cosmogenic nuclide samples proposed for the Isle of Man form a coherent group in the south of the Island and a strong altitudinal gradient from 480m to 135m. There have been no previous attempts to obtain CN ages for the Isle of Man. Second success of the day, was Dan finding his kettlehole, perhaps not unexpected though given there are two on that stretch of coast with very similar stratigraphy. With the cosmogenic samples in the boot, Dave took his leave and departed for the UK.

Day 4 Thursday – the Central Valley, Kirk Michael and Orrisdale: With Dave gone, OSL sampling was very much to the fore. First up the Central Valley of the Isle of Man extending Peel in the west to Douglas in the east, where geomorphology shows moraine ridges arcing north and northeast indicating penetration of ice from the coast. The Ballaharra sand and gravel quarry shows a 12m sequence comprising basal 12-4m gently dipping fore-set planar sands and massive stratified gravels overlain by an upper (4-0m) top-set channel of horizontally stratified gravels with interbeds of planar and planar rippled sands. Western sectors of the current exposures are dominated by glacial diamicts and testify to an ice marginal setting. The sequence described is an ice proximal delta, with an ice contact slope immediately behind the worked exposures (samples 6 and 7). The late morning, saw a confrontation with high tides, the tides won. Slightly later, we began our run through the three retreat stage formations exposed on the Northern Plain of the Isle of Man. First Shellag Formation outwash at Kirk Michael (sample 8), with us filling the time taken to collect gamma dosimetry with sample the Kirk Michael (KM3/4) kettlehole deposits for our tephrachronologists to search for Icelandic volcanic ash layers. The KM3/4 kettlehole includes a basal cold stage lake muds that predate the lateglacial warming (sample 9). The Orrisdale Formation on the Island is quite well dated, with Ian Thrasher’s research, but we selected the northern most sandur trough in the sequence for further work (sample 10-11).

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Day 5 Friday – Jurby Readvance and the Dog Mills: The final day of OSL sampling, we tackled the Jurby Readvance, with two good lithofacies in off-lapping readvance over-ride sequence 3 (samples 12-13), just below a phenomenally well exposure kettlehole, including a prograding delta into the basin (one for the Quaternary community to revisit). The last sample of the day, on the east coast, the Dog Mills proglacial lagoonal sands (sample 14). Thus the sampling over 4-5 days spans the entire retreat sequence on the Isle of Man and two readvance episodes.

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Day 6 Saturday – Bride and seeing what you can do with GPR: With everything complete samples wise, the spare day was just that and with a 19.30 hours departure giving us some leisure time….. What do two Quaternary Geologists with a day spare? Well with 2x GPR antennae and a GPS set up, we assess the performance of GPR for Irish Sea glacigenic lithologies using the Bride Moraine, arguably one of the best if not the best exposure of glacitectonics on the NW European Archipelago. Do we need to know the internal structure of Bride?; well we could just go and look at the 60-80m high cliff sections or read a GSP Thomas paper for that. Again with helpful landowners guiding the way, we gained access to the cliff-tops above Bride, and surveyed 2.5km of the most undulating glacigenic terrain you could hope to meet. The very promising results in hand; we then also set sail for home…..

Another day and another quarry, but this time the BriticeChrono Terrestrial team Rich Chiverrell and Matt Burke met up with some friends, with luminescence dating team Geoff Duller and Holly Wynne from Aberystwyth and stratigraphic geru Geoff Thomas to tackle the delights of rural Cheshire, Transect 3. Breaking all the rules for BriticeChrono quarry investigations the sun was out and not a snowflake in sight or site for that matter. Cherry Orchard Farm is one of a series of sand and gravel quarries to the east of the mid-Cheshire Sandstone Ridge, recently sampled for cosmogenic nuclide (CN) dating.

The site (location 17 on the map) makes an intriging pair with cosmogenic nuclide location ‘Urchin’s Kitchen’ (location 16), a deeply incised bedrock channel eroded subglacially. We hope to compare the performance of luminescence and CN dating techniques with pairings like this. The setting contains numerous the active and former sand and gravel extraction sites around Delamere Forest, and is located on an extensive (8x5km) gently undulating triangular terrace or bench raised >10m above the floodplains of the Weaver Basin. The terrace is fed by channels flowing from the Sandstone Ridge and presumably a former ice margin on the southern edge of zone 5 (see the map).

Quarry operator (Richard Wilding) was fanastically co-operative and allowed us full access to the sections which reveal shallow water sandur and fine-grained glaciolacustrine sands. The sands were a dream to sample, well sorted, stratified, the right grain size for luminescence dating and with excellent exposure throughout the section. Four samples were taken arrayed vertically through the sequence, though probably almost identical in age given the depositional environment, we sampled different lithofacies or depositional environments. The lengthly process was completed in 5 hours, it takes 60 minutes to record the gamma dosimetry (with a field gamma spectrometer) for each sample, which gave plenty of time for discussion, strategy and logging. Then for some differing journeys home, it can’t take that long to drive to Aberystwyth can it?! Can’t wait for the dates and the next phase of sampling on transect 3…..

Taking advantage of the improving weather and as the snows subside Rich Chiverrell and Matt Burke were joined by Derek Fabel and David Small (University of Glasgow) in getting the terrestrial sampling programme underway for BRITICE-CHRONO. The project a NERC-funded consortium that aims to constrain the rate of collapse of the last British-Irish Ice Sheet. The project will employ a number of dating techniques – Optically Stimulated Luminescence (OSL), Radiocarbon (14C), and Terrestrial Cosmogenic Nuclide (TCN) dating – in order to document the retreat of all major ice streams that drained the largely marine-based ice sheet. Terrestrial sampling along the Irish Sea East transect is now well underway after three days of intensive rock removal along the Mid-Cheshire Ridge for TCN dating. Although renowned for its flatness, the rolling farmland of Cheshire/Shropshire is broken by the enormous Mid-Cheshire ridge that reaches a whopping 227 m asl and is, conveniently, aligned roughly perpendicular to the retreating ice margin. As the ridge has been eroded by overriding ice and is cut by deeply incised meltwater channels it presents a great opportunity for us glacial geologists to do our thing: clamber around, stare at, chisel away at, and rest upon various rocks!

Whilst Rich was enthusing year 2 undergraduates about the joys of glacial geomorphology, Matt, David and Derek began the day by braving the wilds of the Wirral with a stroll around Thurstaston Common in order to take a look at the ‘controversial’ Thor’s rock: a site of much debate as to the origin of the numerous erosion marks that cover its surfaces. Some believe the marks record scalloping by glacial meltwater, a plausible argument given the rock sits within a meltwater channel, yet the obvious steps and chutes down its flanks are probably testament to the alternate hypothesis that these marks simply record 100+ years of children climbing over and sliding down the rock!

Meltwater or scallies: which was the greater erosive agent at Thor’s Rock?

After careful consideration we decided to sit on the fence, concluding that many of the marks were originally scoured by meltwater, but have now been enhanced by the locals (us included) and so we decided to sample at a less controversial site: Thurstaston Hill. From here the team drove a couple of miles to Barnston Dale where we were joined by Rich, resisted the temptation of a 10 am pick-me-up at the Fox and Hounds, and hacked a piece of rock from the Barnston meltwater channel.Next stop Urchin’s Kitchen at Primrose Hill Wood. Following what seems to be a tradition for this transect, Rich and David did an excellent job of navigation (Rich: “we don’t need a map to navigate…”).

C3W film crew capturing Derek’s activities

Although we did eventually arrive at the site, we seemed to take the scenic route as we zigzagged our way across the countryside, which was all part of the plan, of course. On site we met up with SaskiaPagella and Vince Jones from C3W who filmed the sampling procedure and interviewed Rich and Derek with the eerie backdrop of the Urchin’s Kitchen.

Unfortunately, although Derek gave us a great recital of a poem commemorating the discoverer of cosmic rays, it was not captured on film and it was a one-off performance. Lesson learned; always have a video camera at the ready in case of any future Britice-Chrono poetry recitals….. After a rather late lunch at Delamere Forest, the team finished off northern sector sampling by bagging (literally) rock at Manley Knolland Helsby Channel. All in all, a very successful day of sampling, but could we cope without Derek on day 2?

DAY 2:Central sector

Afraid to look down: David measures the shielding at Raw Head.

As Derek whizzed off to Sheffield to give a talk, Rich, David and Matt were left to pick up the pieces and continue with another round of TCN sampling. This time the central part of the ridge was the target, where surprisingly little bedrock was exposed. However, after several hours of rock hunting, David stepped into Derek’s shoes admirably and showed his skills at Raw Head and Bickerton Hill. Raw Head proved particularly challenging (not including having to avoid the local fox hunt on route) given David and Matt had to clamber atop a block that had slumped from the main outcrop and was hanging precariously above the valley below. After a long and cold morning of walking the ridge, we were met for lunch by Geoff Thomas at the aptly-named pub “The Sandstone”. Along with a rather large lunch, Rich crumbled under the slightest of peer-pressure to enjoy a pint of “Scrum Down” (BRITICE-CHRONO approved beer 1) with the rest of us.Luckily, after lunch the fully-awake and motivated team were able to expend their excess energy with a couple of steep hikes. The first proved fruitless as we discovered on arrival at the outcrop that it had been heavily quarried. Thankfully, the second hike to the head of the very deeply incised meltwater channel that is Peckforton Gapproduced something we could sample. After another successful day, Matt was finally able to break the habit and navigate us home without any major detours.

Excellent lunch stop

Deeply incised meltwater channel at Hawkstone Park.

Cautious bedrock sampling at Hawkstone Park.

DAY 3:Southern sector

Ahead only lowland Shropshire and the Severn basin

Day 3 began with a long drive to the most southern extent of the ridge in Shropshire where the team were given unrestricted and free access to Hawkstone Park, despite it being closed to the public. After Geoff successfully found his way to the chute-like meltwater channels, the question was asked; have these actually been eroded by meltwater or were they excavated to produce nice walking routes for all those Victorian tourists? After careful consideration, we decided they were in fact meltwater channels, but unfortunately their steep walls and the resulting shielding meant TCN sampling was not possible. Instead, more precarious sites were chosen right at the top, and at the edge, of the escarpment. With bushes on one side and the prospect of quite drop a long fall on the other, Derek and David held their nerves to bag another two good samples. No fieldwork thus far appears complete without without a visit to the pub, the team stopped for another well-deserved lunch at “The Inn at Grinshill” where, alongside very nice fish and chips, a further sample was taken (“Six Nations” –BRITICE-CHRONO approved beer 2). After lunch the sun was shining and we were able to bag the final sample of the day (and the trip) from The Cliffat Corbett Wood. This proved to be a fitting conclusion to the trip: all TCN samples for the transect have now been collected, and the final site at the most southern tip of the ridge overlooked the outwash plains that will be now targeted for OSL dating over the coming weeks…

A new paper has just been published by Richard Chiverrell and a hefty team of Britice-Chrono co-workers (James Scourse, Katrien van Landeghem, Chris Clark, Colm O Cofaigh, Dave Evans, Danny Mccarroll, Colin Ballantyne) presenting the first Bayesian integration and modelling of all the dating control for the marine sectors of the largest ice stream that the last British-Irish Ice Sheet ~ 24,000 years ago. The modelling shows very rapid retreat for this marine-terminating ice stream over greater distances (650 km) and timescales (8000 years) than is available from short term (decadal) observations of present day ice stream margins. The modelling shows this retreat 24,000 years ago was rapid and linked with climatic warming, sea-level rise, mega-tidal amplitudes and reactivation of meridional circulation in the North Atlantic. But, significantly the pattern of retreat appears uneven with a pulsed pattern of retreat attributed to the passage of the ice stream between normal (sloping away from the ice margin) and adverse (sloping towards) ice bed gradients and changes in the geometry or marginal constriction of the ice stream. To read more click here.

The methodology and application kind of formed an important test case for Britice-Chrono as we attempt to constrain rates of and controls on marine ice stream retreat over millennial timescales for eight ice stream radiating out from the last British-Irish Ice Sheet. The methodology outlined in the paper will underpin and be used as a guide for our data collection for the wider British-Irish Ice Sheet. It would be quite good fun to play around with some of the available chronology for other ice streams…..

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